1. Field of the Invention
The present invention relates to a process of a semiconductor fabrication system and for controlling the distribution of dopants or impurities in a device. More specifically, the present invention relates to a technique for controlling punchthrough in a semiconductor device using transient-enhanced diffusion.
2. Description of the Related Art
Punchthrough is a phenomenon associated with merging of source and drain depletion regions in a short-channel device. As the device channel is reduced in size, the distance between depletion regions falls. Punchthrough occurs when the channel length is reduced to approximately the sum of two depletion widths. Punchthrough occurs when the gate voltage is lower than the threshold voltage V.sub.T and occurs as a result of widening of the drain depletion region when the reverse-bias voltage on the drain is increased. Punchthrough occurs if the electric field of the drain eventually penetrates into the source region and reduces the potential energy barrier of the source-to-body junction. The majority carriers in the source region increase in energy sufficiently to overcome the potential energy barrier and an increase current flows from the source to the body. Punchthrough is suppressed by limiting the total width of the two depletion regions to less than the channel length.
Subsurface punchthrough is a phenomenon occurring in submicron N-channel MOSFETs arising from a threshold voltage V.sub.T adjust implant for raising the doping of surface channel region above the lighter doping of the substrate. The implant causes narrowing of the source/drain depletion regions near the surface but does not help in the region beneath the surface of the substrate due to lighter doping beneath the surface. Accordingly, punchthrough first occurs in the subsurface region. One technique for preventing subsurface punchthrough is to increase the substrate doping, thereby decreasing depletion layer widths. A second technique for preventing subsurface punchthrough is implantation of a punchthrough stop implant at a depth near the bottom of the source-drain regions. The punchthrough stop implant reduces the lateral widening of the drain-depletion region below the surface without increasing the doping under the junction regions. The punchthrough stop implant is effective if the placement and dosage is tightly controlled and the punchthrough implant profile is prevented from spreading appreciably during annealing. A third technique for preventing subsurface punchthrough involves local implanting of substrate-type dopants under the lightly-doped tip region of the LDD in a "halo"-type implant. The halo implant raises the dopant concentration only on the inside walls of the source/drain junctions so that the depletion length is decreased without usage of a higher-doped substrate.
Conductive regions of N-type and P-type conductivities and N-P junctions at the boundaries of conductive regions are essential for providing electrical functionality of semiconductor devices and for determining operational characteristics such as punchthrough susceptibility. The conductive regions and junctions are formed in a semiconductor wafer by diffusion or ion implantation techniques. The N-P and P-N junctions in a semiconductor substrate form structures that provide the electrical functionality of transistors and diodes. A junction is a separation between a region called an N-type region having a high concentration of negative electrons and a region called a P-type region having a high concentration of holes. A junction is typically formed in a semiconductor wafer by thermal diffusion, or ion implantation and annealing.
The structure and concentrations of dopants and impurities in the substrate determine the punchthrough performance of a device. What is needed is a technique for implanting and diffusing dopants and impurities that improves the punchthrough susceptibility of semiconductor devices.